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Acta Polytechnica Hungarica
Vol. 4, No. 3, 2007
Chaos and Natural Language Processing
Marius Crisan
Department of Computer and Software Engineering
Polytechnic University of Timisoara
V. Parvan 2, 300223 Timisoara, Romania
Tel.: +40256403254, E-mail: [email protected], http://www.cs.upt.ro/~crisan
Abstract: The article explores the possibility to construct a unified word feature out of the
component features of letters. Each letter is modeled by a different attractor and finally
embedded in a quadratic iterated map. The result is the word feature that can account for
the meaning extraction process of language understanding. This is a new approach in
natural language processing based on the deterministic chaotic behavior of dynamical
systems.
1
Introduction
There is an increased interest in the modern era for developing techniques for both
speech (or character/word sequences) recognition and synthesis. Natural language
processing (NLP) provides those computational techniques that process spoken
and written human language. An important class of methods for language
recognition and generation is based on probabilistic models, such as N-grams
model, Hidden Markov and Maximum Entropy model [1]. Given a sequence of
units (words, letters, morphemes, sentences, etc.) these models try to compute a
probability distribution over possible labels and choose the best label sequence.
Another approach in NLP is to use neural networks, in particular self-organizing
maps of symbol strings [2]-[4]. However, an important challenge for any NLP
approach which may hinder its success is dealing with the nonlinear character of
language phenomenon. Starting from the premise that natural language
phenomena can be viewed as a dynamical system the purpose of this work is to
investigate the possibility of modeling words/characters by a chaotic attractor.
2
Meaning as Wholeness in Dynamical Systems
We may consider consistently with other theories of language that the notion of
word is the constituent element of a sentence (utterance). We might, at first, also
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Chaos and Natural Language Processing
consider in a general formalization that a word is any sound-sequence that
possesses the property of inflection. Normally, each word takes either a verbal,
i.e., conjugational inflection, in which case it is called a verb, or a nominal, i.e.,
declensional inflection, in which case it is of a non-verbal category (substantives,
adjectives, participles, etc.). All the other words which do not have declensional
inflections, such as prepositions, may be considered to possess invariant
inflection. However, to classify words only in terms of their inflection property is
an incomplete task, and does not seem to help much in explaining how the
meaning as structured information is conveyed by a sentence. Therefore, I suggest
the employment of semantic criteria in defining the notion of word. According to
such a view, a word is the meaning-bearing element of a sentence. The semantic
criterion determines the minimum sequence length of the phonemes which convey
a meaning. Thus, words may vary in complexity, from the shortest meaningbearing ones to the more complex compound words. Based only on meaningful
words, we may define, in general terms, a sentence as being a cluster of words
capable to generate a cognitive meaning in an ideal receiver (hearer/reader). This
cognition is a result of a reaction mechanism triggered by the series of words in
the sentence.
An ideal receiver is qualified by the ‘capacity’ to extract meaning from a sentence.
This capacity can be described by the cognition of four cognitive properties that
have been assigned by the transmitter (speaker/writer) to a sentence: (1) semantic
competency, (2) expectancy (syntactic/semantic), (3) contiguity in space and time,
and (4) transmitter’s intention [5]. These cognitive properties are the requirements
for defining a grammatical and meaning-bearing sentence. A sentence is said to
have semantic competency when the objects denoted by the respective words are
compatible one to another. For instance, the sentence ‘He sees the light.’ is
grammatically acceptable, and has semantic competency, while the sentence ‘He
hears the color.’ even if it is grammatically acceptable, lacks semantic
competency. Semantic expectancy refers to the capacity of an ideal receiver to
infer the meaning of an incomplete sentence (utterance). Syntactic expectancy
refers to the syntactic property x which has o be assigned to a sentence s when it is
not grammatical, in order to make it suitable to transmit the meaning. This
expectancy is measured by the predictor of the entropy of the entropic source.
Contiguity is the property which imposes the absence of any unnecessary spatial
(in written text) or temporal (in uttered) interval between the words of a sentence.
In a previous work [6], in defining meaning as something that must have a finite
description, I introduced the concept of undivided meaning-whole (UMW). This is
conceived as structured information which exists internally at the agent’s
information level. Even if UMW is a unitary information structure, it is
describable rationally in terms of cognitive semantic units. These semantic units
are the generating principle of producing the sequence of uttered words.
When an agent wants to communicate, it begins with the UMW existing internally
in its knowledge base. When words are uttered producing different sounds in
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Vol. 4, No. 3, 2007
sequence, it appears only to have differentiation. Ultimately, the sound sequence is
perceived as a unity or UMW and only then the word meaning, which is also
inherently present in the receiver’s mind, is identified.
The above described capacity of the receiver to extract meaning from series of
words led to another assumption, that the whole word/sentence meaning has to be
inherently present in the mind of each agent. Thus, it can be explained how it is
possible the UMW to be grasped by the hearer even before the whole sentence has
been uttered. The sounds which differ from one another because of difference in
pronouncement cause the cognition of the one changeless UMW without
determining any change in it. Sometimes, reasoning may have to be applied to the
components of the sentence so that the cognition is sufficiently clear to make
possible the perception of the meaning-whole. It appears that the unitary wordmeaning is an object of each person’s own cognitive perception. When a word,
such as ‘tree’ is pronounced or read there is the unitary perception or simultaneous
cognition of trunk, branches, leaves, fruits, etc. in the receiver’s mind.
Communication (verbal or written) between people is only possible because of the
existence of the UMW, which is potentially perceivable by everybody and
revealed by words’ sounds or symbols.
The concept of UMW is consistent with a more general view, suggested by Bohm
in [7], regarding the possibilities for wholeness in the quantum theory to have an
objective significance. This is in contrast with the classical view which must treat
a whole as merely a convenient way of thinking about what is considered to be in
reality nothing but a collection of independent parts in a mechanical kind of
interaction. If wholeness and non-locality is an underlying reality then all the other
natural phenomena must, one way or another, be consistent with such a model.
Natural language generation and understanding is a phenomenon that might be
modeled in such a way. UMW is like ‘active information’ in Bohm’s language,
and is the activity of form, rather than of substance. As Bohm puts it clearly [7],
‘…when we read a printed page, we do not assimilate the substance of the paper,
but only the forms of the letters, and it is these forms which give rise to an
information content in the reader which is manifested actively in his or her
subsequent activities.’ But, similar so called mind-like quality of matter reveals
itself strongly at the quantum level. The form of the wave function manifests itself
in the movements of the particles. From here, a new possibility of modeling the
mind as a dynamical system is considered.
In line with Kantian thought, in [8] we find a similar insight, as above, regarding
the linguistic apprehension. This is the interplay of two factors of different levels:
(1) the empirical manifold of the separate letters or words and (2) the a priori
synthesis of the manifold which imparts a unity to those elements which would
otherwise have remained a mere manifold.
According to this kind of observations it appears motivated to use the concept of
manifold for modeling the mind as the seat of language generation and
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Chaos and Natural Language Processing
understanding. Manifolds are defined as topological spaces possessing families of
local coordinate systems that are related to each other by coordinate
transformations pertaining to a specific class. They may be seen also as the
multidimensional analogue of a curved surface. This property seems suitable to
represent both the natural language constraints and semantic content of linguistic
objects.
Usually, a dynamical system is a smooth action of the reals or the integers on a
manifold. The manifold is the state space or phase space of the system. Having a
continuous function, F, the evolution of a variable x can then be given by the
equation:
xt+1 = F(xt).
(1)
The same system can behave either predictably or chaotically, depending on small
changes in a single term of the equations that describe the system. Equation (1)
can also be viewed as a difference equation (xt+1 – xt = F(xt) – xt) and generates
iterated maps. An important property of dynamical systems is that even very
simple systems, described by simple equations, can have chaotic solutions. This
doesn’t mean that chaotic processes are random. They follow rules, but even the
simple rules can produce amazing complexity. In this regard, another important
concept is that of an attractor. An attractor is a region of state space invariant
under the dynamics, towards which neighboring states in a given basin of
attraction asymptotically approach in the course of dynamic evolution. The basin
of attraction defines the set of points in the space of system variables such that
initial conditions chosen in this set dynamically evolve to a particular attractor. It
is important to note that a dynamical system may have multiple attractors that may
coexist, each with its own basin of attraction [9]. This type of behavior is suitable
for modeling self-organizing processes, and is thought to be a condition for a
realistic representation of natural processes.
One example of such an approach is the topological feature map proposed by
Kohonen [10], [11] for the projection of high dimensional pattern data into a lowdimensional feature space. The process of ordering an initial random map is called
in this approach self-organization. The result is the topological ordering of pattern
projections, or in other words the self-organizing map (SOM). Each input
dimension is called a feature and is represented by an N-dimensional vector. Each
node in the SOM is assigned an N-dimensional vector and is connected to every
input dimension. The components or weights of this vector are adjusted following
an unsupervised learning process. First, it is found the winning node, i.e, the node
whose weight vector shows the best match with the input vector in the Ndimensional space. Next, all weight vectors in the neighborhood in the direction
given by the input vector are adjusted. This process requires many iterations until
it converges, i.e., all the adjustments approach zero. It begins with a large
neighborhood and then gradually reduces it to a very small neighborhood.
Consequently, the feature maps achieve both ordering and convergence properties,
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Vol. 4, No. 3, 2007
and offer the advantages, of reducing dimensions and displaying similarities.
However, SOM solutions (and neural networks in general) are yet in the need for
improvement. For instance, in [12], an important problem for SOM is discussed.
In order to obtain a realistic speech projection, the problem is to find a
hypercubical SOM lattice where the sequences of projected speech feature vectors
form continuous trajectories. In another work [13], both SOM and a supervised
multilayer perceptron were used for bird sounds recognition. The conclusion was
that although the tested algorithms proved to be quite robust recognition methods
for a limited set of birds, the proposed method cannot beat a human expert
listener.
On the other hand, the unexplored domain of dynamical systems and chaos theory
may offer promising perspectives in modeling natural processes, and NLP might
be one of them.
3
Attractor-based Word Modeling
In quantum experiments, when particles interact, it is as if they were all connected
by indivisible links into a single whole. The same behavior is manifested by the
chaotic solutions in an attractor, as we will see in this section. In spite of the
apparent random behavior of these phenomena, there is an ordered pattern given
by the form of the quantum wave (or potential) in the former case, and by the
equations of the dynamic system in the latter.
Let’s consider the simplest case of the quadratic iterated map described by the
equation:
xt+1 = a1 + a2xt + a3xt2
(2)
Even if it is so simple, it is nonlinearly stable and can manifest chaotic solutions.
The initial conditions are drawn to a special type of attractor called a strange
attractor. This may appear as a complicated geometrical object which gives the
form of the dynamic behavior.
In nonlinear dynamics the problem is to predict if a given flow will pass through a
given region of state space in finite time. One way to decide if the nonlinear
system is stable is to actually simulate the dynamics of the equation. The primary
method in the field of nonlinear dynamic systems is simply varying the
coefficients of the nonlinear terms in a nonlinear equation and examining the
behavior of the solutions. The initial values of the components of the model
vector, mi(t), were selected at random in a process of finding a strange attractor.
Strange attractors are bounded regions of phase space corresponding to positive
Lyapunov exponents. We found more than 100 attractors. In Table I we presented
a list of several coefficients along with the Lyapunov exponent for which the
attractors were found by random search. The initial condition x0 was selected in
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Chaos and Natural Language Processing
the range 0.01 – 1 and lies within the basin in many cases. The Lyapunov
exponent is computed in an iterated process according to the following equation
[14], [15]:
LE = Σlog2 |a2 + 2a3xt| /N
(3)
The sum is taken from a value of t = 1 to a value of t = N, where N is some large
number.
Table 1
The coefficients values and the Lyapunov exponent for 25 attractors of (2)
Cur. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
a1
1.2
1.2
1.1
1.1
1.1
1
0.9
0.9
0.8
0.8
0.7
0.7
-1.2
-1.2
-1.2
-1.1
-1.1
-1
-0.8
-0.8
-0.7
-0.7
-0.6
-0.5
-0.4
a2
-1
-0.2
-1.2
-1
-0.6
-0.7
-1.1
-1.1
-1.2
-0.9
-1.2
-1.1
-1.2
-0.9
-0.6
-0.8
-1
-1
-1.1
-1.1
-1
-1.2
-1.2
-1.1
-1.2
a3
-1
-1.1
-0.8
-0.6
-1
-1.1
-1.1
-0.8
-1.2
-1
-0.8
-1.2
0.7
0.8
1
1
0.9
0.7
1.1
0.7
1
1
1.2
1.2
1.2
LE
0.4235
0.1198
0.3564
6.6073
0.1443
0.2512
0.3571
0.256
0.411
0.1383
0.2001
0.3029
0.2918
0.2793
0.2662
0.286
0.3054
0.1209
0.3047
6.9382
0.1248
0.285
0.2801
0.1375
0.1344
LE gives the rate of exponential divergence from perturbed initial conditions. If
the value is positive (for instance, greater than 0.005) then there is sensitivity to
initial conditions and a strange attractor can manifest. If the solution is chaotic, the
successive iterates get farther apart, and the difference usually increases
exponentially. The larger the LE, the greater is the rate of exponential divergence,
and the wider the corresponding separatrix of the chaotic region. If LE is negative,
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Vol. 4, No. 3, 2007
the solutions approach one another. If LE is 0 then the attractors are regular. They
act as limit cycles, in which trajectories circle around a limiting trajectory which
they asymptotically approach, but never reach.
Figure 1
Quadratic iterated map of (2)
It’s interesting to analyze in more details the behavior of an attractor. The idea of
the self-organizing maps is to project the N-dimensional data into something that
is better understood visually. A similar idea we follow in constructing iterated
maps. It is convenient to plot the values in the iterated process versus their fifth
previous iterate for a more suggestive aspect. In Fig. 1 it is presented the iterated
map for the strange attractor No. 3. A remarkable property of the chaotic
solutions, as noted above in connection with quantum, is the ‘ballet-like’ behavior
as iterations progress. Each new dot on the map, representing the solution xt+1,
appears in a random position but orderly following the attractor’s form.
In Fig. 2 it is shown the same attractor only after a few iterates (2000). It can be
seen the sparse distribution of dots but along with the ordered path. This type of
behavior is similar with the quantum phenomena, such as the distribution of
photons along the interference pattern lines in the two slit interference experiment,
when the photons are emitted in series one after the other. This is also akin to the
quality of the perception act (word meaning). It is observed that a word meaning is
at first perceived vaguely and then more and more clearly. Thus, through the
process of repeated perception or iterations finally the meaning is revealed. We
may suggest, therefore, that meaning can be mathematically modeled as a basin of
attraction.
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Chaos and Natural Language Processing
Figure 2
Quadratic iterated map of (2) after 2000 iterates. Note the sparse distribution of dots along the regular
pattern of the strange attractor.
Another interesting property is the symmetry of a1 and a3 and the corresponding
iterated map. Considering again the strange attractor a1 = 1.1, a2 = –1.2, a3 = -0.8,
a symmetric behavior can be obtain for the values a1 = –1.1, a2 = –1.2, a3 = 0.8 as
in Fig. 3.
Figure 3
The symmetric quadratic iterated map of Fig 1, obtained by inversing the sign of a1 and a3
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Vol. 4, No. 3, 2007
There is a huge possibility to obtain other attractors by tuning the values of the
coefficients. The shape of the attractor changes smoothly with small variations of
the coefficients. Even if the interval of variation is rather small, visible changes in
the shape of the map can be obtained. For instance, if a1 = 1.02 the value of LE is
0.09 and the limit cycles can be observed as the attractor becomes regular. If a1 =
1.3 regular oscillations are manifested.
Figure 4
The completely changed quadratic iterated map of (2), obtained for a2 = –1.005
An important change in shape can be obtained by modifying a2. The value for a2
always has to be negative for a bounded behavior. For a2 = –1.005 the shape of the
map is drastically changed as shown in Fig. 4.
4
Language Recognition
One widely used method to classify an object is to measure its features
(characteristic properties). In general the features that are to be observed depend
on the specific problem one has to solve. In language recognition, we deal with
several kinds of features such as graphological, phonological, statistical, syntactic,
lexical, semantic, and pragmatic. Graphological features are for instance letter
positions and word shape. Phonological features are considered as the distinctive
features from which phonemes can be constructed [16]. The syntactic features are
present in the construction of words and sentences, and are part of speech tags and
various components from a parse tree. Statistical features exploit the fact that
more frequently occurring words are more familiar and hence more easily
recognized. These features may be the frequency of occurrence of letters, letter
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Chaos and Natural Language Processing
pairs and triplets, the average word length, the ratio of certain characters, word
endings, consonant congestion [17], etc. Lexical features are used to represent the
context. They consist of unigrams, bigrams (a pair of words that occur close to
each other in text and in a particular order), and the surface form of the target
word which may restrict its possible senses [18]. Semantic features indicate the
meaning of words, and are usable for disambiguation of words in context.
Pragmatic features are based on how the words are used. In general, the above
mentioned features are tried to be described by morphology using the concept of
morphemes or the constituent parts of words.
Irrespective of the feature’s nature, the result of feature extraction or measurement
is a set described as an n-dimensional feature vector associated with the observed
object, which can thus be represented as a point in the n-dimensional feature
space. Next, a classifier will assign the vector to one of several categories. While
the use of features has a central place in pattern classification [19], the design and
detection of features in natural language remains a difficult task because of
language high complexity and the lack of a unitary theory.
The analysis in the previous section revealed the fact that attractors offer dynamic
properties that can map in a continuous manner the feature vectors according to
some input patterns. Considering the assumption of UMW, the goal is to construct
a unified word feature that might account for the word meaning. I propose a
possible non-linear many-to-one mapping from a conventional feature space to a
new space constructed so that each word has a unique feature vector. Let’s
consider the simpler case of a 3-dimensional feature vector characterizing a letter.
The vector for a generic letter ‘A’ is defined by the values a = [a1, a2, a3], and
similarly for the generic letters ‘B’ and ‘C’ the vectors are b = [b1, b2, b3], and c =
[c1, c2, c3] respectively. The letter feature of ‘A’ results in an iterated process as
At+1 = a1 + a2At + a3At2,
(3)
starting from an initial condition A0.
Similar equations result for the letter features of ‘B’ and ‘C’, with the initial
conditions B0 and C0 respectively, as the following:
Bt+1 = b1 + b2Bt + b3Bt2,
(4)
Ct+1 = c1 + c2Ct + c3Ct2.
(5)
Based on letters features, for each letter in a word (for instance with length 3) a
unified feature vector W = [A, B, C] can be constructed and mapped to the three
coefficients of an equation of type (2). The result is of the following form:
Wt+1 = At + BtWt + CtWt2.
(6)
Eq. (6) is computed starting from an initial condition W0 and manifests a chaotic
deterministic behavior for a proper combination of the coefficients A, B, and C. In
Fig. 5 it is presented the iterated map of (6) for the input vectors a = [0.8, –1.2, –
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Vol. 4, No. 3, 2007
0.9], b = [–1, –0.9, 1.1], and c = [1.1, –1.2, –0.8] after 5000 iterations, and the
initial condition for all parameters of value 0.01. In order to have a suggestive
view of the unified feature space and observe its internal structure, the values were
plotted versus their third previous iteration. Also, the values of W were bounded to
108 for a convenient screening. The same sparse distribution of dots along the
regular pattern of the feature space, typical for deterministic chaos, can be
observed as the iterations progress.
Figure 5
The chaotic deterministic behavior of (6) for the letter feature vectors a = [0.8, –1.2, –0.9], b = [–1, –
0.9, 1.1], and c = [1.1, –1.2, –0.8]
Each valid word of length 3 will determine a corresponding iterated map. Small
variations in the input will be tolerated and recognized with the same meaning but
other illegal combinations will be rejected. For instance, in Fig. 6 we can see the
feature space for a rather consistent deformation of the input vectors b = [–1.3, –
0.6, 1.3] and c = [.9, –1.3, –1].
Comparing the feature spaces of Fig. 5 and Fig. 6 we can observe the vague
resemblance between the two, and after a closer examination we can identify in
fact a similar chaotic pattern. This means that the meaning was conserved even if
some visible alterations affected two of the letter features. If the changes are more
dramatic we expect a completely different pattern or even an unbound behavior.
This indicates the lack of properties for a meaningful word. In Fig. 7 it is
presented the case where the vectors a and c swaped their contents. This means
another word where the first and last letters are interchanged.
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Chaos and Natural Language Processing
Figure 6
The feature space of (6) for a = [0.8, –1.2, –0.9], b = [–1.3, –0.6, 1.3], and c = [.9, –1.3, –1]. Note
the vague resemblance with Fig. 5.
Figure 7
The chaotic deterministic behavior of (6) for the letter feature vectors a = [1.1, –1.2, –0.8], b = [–1, –
0.9, 1.1], and c = [0.8, –1.2, –0.9]
A completely different pattern is obtained comparing to Fig. 5. Of course,
depending on the classifier conventions, the pattern can be meaningful or not. In
any case, it represents the unique feature vector for that word construction.
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Vol. 4, No. 3, 2007
For words with higher length, higher-order iterated maps can be used. The
proposed approach can be extended for a whole sentence. In this case, the unified
feature vector of the sentence is constructed based on the features of the individual
component words. This will be the UMW equivalent of the whole sentence.
Conclusions
Our purpose was to study the possibility of using dynamical systems in modeling
natural language processing. We started from the premise of UMW and the
observation facts of language apprehension and noted a similitude with the chaotic
behavior of dynamical systems. The attractor behavior as was studied for the
quadratic iterated maps seems to be robust enough to model the feature vectors
formed for each word of length 3 in the dictionary. The unified word feature
vector is obtained by a many-to-one mapping, starting from the component letters,
and bears the unique information structure of the word meaning. Slight variations
in the input feature vectors of the component letters are tolerated, without major
changes of the pattern in feature space structure. This is an indication of meaning
preservation in the case of noise. The chaotic deterministic behavior of the
patterns in the feature space may account for meaning recognition process after a
series of repeated perceptions. After enough iterations (or repeated perception) the
attractor shape is recognized and consequently the corresponding meaning. The
present work may be continued in the future by constructing the unified feature
vector at the sentence level.
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